The present invention relates to a novel process and system for eliminating high SO2 emissions associated with Claus unit shutdowns, and the potential environmental regulatory violations associated therewith.
The Claus process is a catalytic chemical process that is used for converting gaseous hydrogen sulfide (H2S) into elemental sulfur (S). This process is widely used to produce sulfur from H2S found in raw natural gas and from by-product sour gas streams derived from petroleum crude oil refinery processes such as amine gas treating and sour water stripping.
Claus unit feed gases have a wide range of compositions, ranging from less than 25 mol % H2S (from a sour water stripper and natural gas treatment facilities) to greater than 80 mol % H2S (from a refinery amine regenerator). The Claus technology can be divided into two process steps, thermal and catalytic. In the thermal step, the feed is combusted to convert H2S into SO2 as follows:2 H2S+3 O2→2 SO2+2 H2O  (1)
In the catalytic step, using activated aluminum (III) or titanium (IV) oxide, H2S reacts with the SO2 formed during the thermal step to form elemental sulfur:2 H2S+SO2→3 S+2 H2O  (2)
The overall process reaction can thus be expressed as:2 H2S+O2→2 S+2 H2O  (3)
The tail gas from the Claus process still contains combustible components and sulfur compounds, and is either burned in an incineration unit or further desulfurized in a downstream tail gas treating unit (TGTU), such as, for example, the Shell Claus Off-gas Treatment (SCOT) technology that uses a Co—Mo catalyst bed followed by an amine treatment section.
It is typically necessary to shut down a Claus unit for inspection or repairs. Because it is common for large sulfur removal complexes to employ a number of parallel Claus units, it would be ideal to allow a single Claus train shutdown without the need to shut down the other parallel Claus trains or common TGTUs. This however has proven to be problematic.
As an example, a sulfur removal plant consists of two Claus units flowing into a single TGTU. It is desired to shut down one of the Claus units while the other Claus unit is processing refinery acid gas, thus preventing the entire shutdown of the sulfur removal complex and altering refinery operations. During the shutdown operation, natural gas is combusted in the Claus reaction furnace to strip sulfur from the Claus reactors. The tail gas from this shutdown operation consists therefore of natural gas combustion products (CO2, H2O, N2), residual sulfur species (S, H2S, SO2, SO3, COS, CS2), with excess O2 concentrations ranging from 0% (stoichiometric natural gas firing at the beginning of the shutdown sequence) up to 21% (pure air at the end of the shutdown sequence).
Referring generally to FIG. 1, a flow diagram is shown illustrating the problems associated with prior art methods for implementing the single Claus unit shutdown described above. While natural gas firing remains at stoichiometric O2 concentrations (i.e., less than 0.5% excess O2), the resultant tail gas is sent to the TGTU for processing. This occurs until sulfur production stops, at which time the tail gas is diverted from the operating TGTU directly to the incinerator and excess O2 concentrations are introduced to the system. One of ordinary skill in the art will appreciate that excess air/O2 can never be present in tail gas sent directly to the TGTU as this would damage the catalyst in the TGTU. As O2 concentrations are increased in the furnace, the SO2 concentration in the tail gas also increases as more sulfur species are removed. In order for the incinerator exhaust gas to remain below environmental regulations (e.g., 250 vppm), the tail gas must then be diverted back to the TGTU (to avoid an environmental exceedance). The process of diverting the shutdown tail gas back and forth between the TGTU and the incinerator is time consuming, and often results in the need to completely shut down the entire sulfur removal plant.
What is still needed is a reliable process improvement that allows the shutdown of one Claus unit while other parallel Claus units remain online processing refinery acid gas.